Convert CONFIG_ETHPRIME to Kconfig

This converts the following to Kconfig:
   CONFIG_ETHPRIME

This is also done by adding a gating Kconfig option, CONFIG_USE_ETHPRIME
similar to other options that are not always set and control environment
variables.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mm_venice_defconfig b/configs/imx8mm_venice_defconfig
index 02a65c5..7cb4a9a 100644
--- a/configs/imx8mm_venice_defconfig
+++ b/configs/imx8mm_venice_defconfig
@@ -62,6 +62,8 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eth0"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_IP_DEFRAG=y
 CONFIG_TFTP_BLOCKSIZE=4096